The present invention relates to a paper feed apparatus in a printer as a terminal output device and, more particularly, to a paper feed apparatus capable of simultaneously feeding a plurality of stacked sheets (to be referred to as copy paper hereinafter) without causing misalignment thereof
It is very convenient to simultaneously obtain a plurality of prints when a printing head strikes a plurality of sheets However, it is difficult to simultaneously feed the sheets at a given speed. Strictly speaking, the radii of curvature of the sheets having arcuated feed paths on the outer surface of a platen differ from each other. A feed amount of the sheet on the platen side differs by a deviation D from that of the sheet on the side opposite to the platen when the sheets are simultaneously fed by the platen. More specifically, feeding of the inner sheet on the platen side is delayed from that of the outer sheet. As shown in FIG. 4, the deviation D between the sheets is defined as follows: EQU D=(n-1)t.multidot.2.pi..multidot.(.theta./360)
where n is the number of sheets constituting copy paper a, t is the thickness of each sheet, and .theta. is the wound angle.
If t=0.1 (mm), n=4, and .theta.=120.degree., then the deviation D=0.62 (mm) is generated every .theta..degree. rotation of the platen. This deviation D is theoretically accumulated when the platen is rotated further. However, according to an observation result, when the deviation D has reached 3 to 4 mm, a given saturation is achieved. Upon achieving a saturation, the sheets are fed while keeping this deviation. During feeding, perforated portions and perfect-bound portions which are provided normally at both sides of the copy paper can endure a tension force difference caused by the above deviation. In addition, elasticity of the paper itself often allows intermittent skips on the contact surface with the platen to prevent an infinite accumulation of deviations.
However, the deviation of 3 to 4 mm causes poor printing on paper. The above-mentioned skips are not allowed depending on the types of printers, and elasticity of the paper is not sufficient depending on the types of paper. Pin holes a' of the inner sheets are gradually deviated from those of the outer sheets, and finally paper jam occurs. Reference numeral 100 in FIG. 4 shows a printing head.
Prior arts for solving problems posed by a plurality of sheets to be printed are Japanese Patent Laid-Open Application Nos. 51-118512, 53-25107, and 54-27808 (to be referred to as prior arts (1), (2), and (3) hereinafter). In prior art (1), the deviation during feeding of printing paper consisting of a plurality of sheets is prevented by zig-zag feeding. In prior art (2), the deviation during feeding of printing paper consisting of a plurality of sheets is prevented after the edge of the paper is trimmed and the edge-trimmed paper is fed to the printing head. In prior art (3), the temporary bound portions of the printing paper consisting of a plurality of sheets are designed not to interfere with the printing head. Therefore, these prior arts presented problems to be solved and means for solving these problems, all of which are different from those of the present invention.